SUCCESSION 271 
to follow. For this, the use of the permanent quadrat is imperative, as the 
sequence and structure of the stages can be understood only by a minute 
study of the shifting and rearrangement of the individuals. Permanent mi- 
gration circles are indispensable for tracing movement away from the pioneer 
areas by which each stage reaches its maximum. Denuded quadrats are a 
material aid in that they furnish important evidence with respect to migration 
and ecesis, By means of them, it is possible to determine the probable devel- 
Fig. 70. Alternating gravel slides on Mounts Cameron and Pals- 
grove, from the comparison of which the initial development of the 
talus succession has been reconstructed. 
opment of stages which reach back a decade or more into the past. In the ex- 
amination of successions, since cause and effect are so intimately connected in 
each reaction, it is especially important that general and superficial observa- 
tions upon structure and sequence be replaced by precise records, and that 
vagtie conjectures as to causes and reactions be supplanted by the accurate 
determination of the physical factors which underlie them. 
329. Method of alternating stages. The period of time through 
which a primary succession operates is usually too great to make a complete 
study possible within a single lifetime. Secondary successions run their 
course much more quickly, and a decade will sometimes suffice for stabiliza- 
